Ion implantation systems are used to dope semiconductors with impurities in integrated circuit manufacturing. In such systems, an ion source ionizes a desired dopant element, which is extracted from the source in the form of an ion beam. The ion beam is typically mass analyzed to select ions of a desired mass/charge ratio and then directed at the surface of a semiconductor wafer in order to implant the wafer with the dopant element. The ions of the beam penetrate the surface of the wafer to form a region of desired conductivity, such as in the fabrication of transistor devices in the wafer. A typical ion implanter includes an ion source for generating the ion beam, a beamline assembly including a mass analysis apparatus for mass resolving the ion beam using magnetic fields, and a target chamber containing the semiconductor wafer or workpiece to be implanted by the ion beam.
In order to achieve a desired implantation for a given application, the dose and energy of the implanted ions may be varied. The ion dose controls the concentration of implanted ions for a given semiconductor material. Typically, high current implanters are used for high dose implants, while medium current implanters are used for lower dose applications. The ion energy is used to control junction depth in semiconductor devices, where the energy levels of the beam ions determine the degree to which ions are implanted or the depth of the implanted ions within the semiconductor or other substrate material. The continuing trend toward smaller semiconductor devices requires a mechanism that serves to deliver high beam currents at low energies. The high beam current provides the necessary dose levels, while the low energy permits shallow implants.
Medium current implantation systems are typically single wafer systems and are capable of high tilt angle implants. Such systems are typically suitable for low and medium dose processes in the medium to high energy range (e.g., 10-250 keV). At low energies, the beam current capability often drops substantially due to poor beam transport efficiency through the various optical elements within the beamline. One technique commonly used to improve the beam transport efficiency is to transport the beam at a high energy, and then decelerate the beam to the desired energy at a short distance from the workpiece. Typically, the higher the deceleration ratio, the more gain in beam transport efficiency.
Deceleration systems such as the one described above, however, sometimes produce undesirable energy contamination at the workpiece. Energy contamination is a condition where neutral particles that are generated within the beam prior to deceleration are allowed to reach the target. Since the neutral particles are not decelerated, such particles reach the workpiece at a substantially higher energy than the rest of the ion beam, and thus are often referred to as energy contaminants.